1. FIELD OF THE INVENTION
The present invention relates to a porous composite structure having enhanced structural strength. In particular, the porous composite structure of the present invention is suitable for fluidization application and for the control of aerodynamic noise.
2. DESCRIPTION OF THE PRIOR ART
There are a variety of porous composite structure which are designed to solve a number of individual problems. For example, in U.S. Pat. No. 3,260,370, issued to Schwartzwalder on July 12, 1966, relates to a composite filter element for use in filtering and conditioning dry cleaning solvents. In another example, U.S. Pat. No. 3,679,062, issued to Burkhart on July 25, 1972, the composite filter element is designed to resist outward distortion to the filter sheets during the backwashing and cleaning of the filter sheets. Thus, while each of the many prior art composite porous structures solve a unique specific problem, most are unsuitable for a variety of applications such as influidizing media and the control of acoustic noise.
Fluidization occurs when either a liquid or a gas, most commonly ambient air, is moved or blown through a dry powder to separate the particles and permit them to behave as a fluid. More exactly, when a fluid is passd upwardly through a bed of closely sized granular particles, a pressure gradient is required to overcome friction. In order to increase the rate of flow, a greater pressure gradient is required. When the pressure drop approaches the weight per unit surface area of the particles, they begin to move and exhibit fluid properties. There are several major devices advantageously using the fluidization of powder material, including gravity conveyors, powder dryers and coolers, batch powder mixers, bulk storage silos, separators and heat transfer beds.
In the case of gravity conveyors, the fluidization process is used to facilitate transporting a dry powdered material a predetermined distance. A porous sheet having a longitudinally formed trough and having a small angle of incline is used as the conveyor. Thus, while the material is fluidized, it flows somewhat like water along the trough. The conveyor may have a very small angle of inclination which is considerably less than the angle of repose of the powdered material because the material is fluidized.
With respect to powder dryers and coolers, the powder material is fluidized in order to evaporate moisture from the powder or to exchange heat through the powder. Batch powder mixers use fluidization to accomplish the mixing of different powders within a batch. The powders become agitated because of the low resistance to fluid flow while fluidized and thus the powders become throughly mixed.
Silos for holding cement powder and for other bulk storage are additional examples of fluidizing applications. To enable easy removal of the powder from the silo, a fluidized bed at the bottom of the silo blows air through the powder to impart to the powder a fluidized state. Once fluidized, the powder can be removed from the silo by simply blowing the powder out of the silo, since the powder acts very much like a liquid.
An example of an application for an illutration separator is separation of grain from impurities such as weed seeds and small stones. In this example, the grain mixture is passed over and moved across a fluidization bed at a very small angle relative to the horizontal. The fluidized mixture is then moved outside of the fluidization bed and is separated by gravity, since the trajectory of the particles will vary. The stones, being heavy, will fall within a comparatively short distance. The grain seeds will fall within a moderately longer distance than the stones while the weed seeds, which are generally much lighter particles than the grain seeds, travel still further.
The use of fluidization in the heat transfer beds consists of placing a part in a fluidization bed of silica or aluminum oxide utilizing hot air as the fluidizing media. The part is heated very rapidly due to the convection effect caused by the motion of the small particles of silica sand or aluminum oxide and the impingement of these particles on the part.
In all of the above mentioned fluidizing applications, porous elements are useful. When choosing a porous element, its strength, its resistance to abrasion and puncture, its ease of cleaning either chemically or by steam, its flow characteristics, its cost, and its ability to operate over a wide range of temperatures are all important considerations.
In addition to the above mentioned considerations, fluidizing applications require that the element have a a strong construction yet have appropriate perforation to provide a high pressure drop uniformly over the element as compared to the pressure drop through the solid particles. This is necessary because, if the pressure drop across the media is similar to the pressure drop across the element, the operation will become unstable which, of course, is undesirable.
One prior art design of a porous element for fluidizing applications utilizes a single plate with a plurality of drilled holes therethrough. It has been found that this design was not satisfactory since the holes weakened the element. Furthermore, it was prohibitively expensive and did not provide the pressure drop characteristics required for fluidizing applications.
In another prior art design of a porous element for fluidizing applications, a layer of wire cloth which has been roll calendered and fusion bonded between two layers of plain mesh weave wire has been found to be moderately successful in some fluidizing media applications. However, an element of this type of construction is also expensive and is difficult to fabricate.
Thus, none of the known art porous element designs provides an inexpensive and easy to fabricate porous media suitable for fluidizing applications.
The control of aerodynamic noise created by gas flow through restrictions and piping systems has also become increasingly important as noise levels in airplanes and in industrial facilities have been subjected to close governmental regulation. A major source of noise in such situations is an aerodynamic phenomenon associated with high velocity flow rate. The high velocity flow rate is created by a rapid expansion of a gas after passing through a flow restriction, thereby creating a localized high velocity flow condition.
To prevent such localized high velocity flow conditions, sophisticated flow path elements have been used to gradually decrease the pressure of the gas so that the velocity remains substantially constant and at a relatively low rate. Such flow path elements may accomplish the desired control of gas velocity, but at a relatively high cost. Furthermore, they limit the flexibility of design because of the varying flow conditions under which these devices must operate. This has led to the consideration of using porous materials. Unfortunately, it is difficult to accurately control pore size in porous materials to prevent localized conditions of high velocity flow created by the currents of relatively small openings.
One such material in which relatively precise control of pore size is obtained is described in U.S. Pat. Nos. 2,457,657 and 3,123,466 and U.S. Application No. 945,261, filed Sept. 22, 1978. This material is formed by a precision winding operation in which wire ribbon material is wound on a mandrel with successive windings being crossed relative to each other to create porous layers having openings of precisely controlled size. The layers of the windings are subsequently diffusion bonded to provide a unitary structure. This approach, however, provides a material which is expensive to make and is susceptible to damage and to plugging of the exposed wire cloth layers.
Accordingly, it is an object of the present invention to provide a controlled porousity composite structure which is adaptable for use in both fluidizing bed and acoustical applications and which is inexpensive and simple to fabricate. The porous composite structure is formed of several porous elements by bonding one layer of weave cloth between two layers of perforated sheets wherein the airflow resistance is controlled by transverse flow paths between the sheets and the cloth with the open mesh transferring stress to the strong perforated sheets.